OK, I know the question sounds a little bit extreme, but I'm writing a story in which there are two races of a same species, both rational and civilized, with cities and everything an actual society has. Both races are pictured as educated and not prone to violence (they're nice, but serious if needed people)

The thing is, both races hate each other, there are people among each race that would go to war if they can and support it, but there is no one who is OK with the other one, like... the most pacifist guy would still sit on the far side of the bus if they see a member of the other race in the front (that was just an explanatory scenario, they don't share cities.)

The thing is, the hate they have on each other must be well founded, it should be so that the reader doesn't think "Oh, there is NO way they can be like that", and it must be about race, not politics or religion. Also, it can't be a historical thing (Like, because of something that happened wayyy in the past) because they have the cultural level and the ethics to let go of the past if that was the case.

I was thinking of maybe something biological, but I don't think a virus or disease might work because I don't think it's possible for they to have evolved to the point where they share a planet on equal grounds (no race is noticeably poorer or richer in resources or education). Although, I'm open to suggestions on other areas aside from disease.

Finally, it can't be simply "nature" or because they are pretty much like humans, they can ignore their nature because they're rational.

EDIT: I'm going to give more details because I feel that some people who answered didn't get an important part of the question (probably my fault, I'm not from an English speaking country)

The idea here is that the reason not only isn't present in our world, but it's most likely that it is impossible that it applies to our world. Many people say in comments that it doesn't have an answer because I'm restricting the answer so it doesn't involve politics, history or religion, but I never said anything about geography, biology, chemistry or any science. Because its hard but very important in the story is why I'm pretty much open for you to do anything with the world, for example, does your reason need for them to live in separate continents? Sure, go for it. Thanks again for everyone who has answered, you have given me a lot of possible reasons I haven't considered.

$\begingroup$You dismiss history as not an option because they'd let go of things "way in the past", but a historic event that causes systematic racism would mean constant examples of racism in the present. Look at the Israel-Palestine relationship. It could be said to have been triggered by the Jewish settling after WW2 (over-simplification) but the conflict has raged on since. Even now I expect many people in both countries have been negatively effected by the constant conflict. It's not a matter of what happened in 1948, it's about what's still happening.$\endgroup$
– CentimaneFeb 2 '17 at 4:54

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$\begingroup$I only have a limited experience, but in every racist society I've heard enough about, not everyone is racist. Every society has people who don't share the common view. A lot of people have suggested answers along the lines of "It is physically unpleasant to be near the other race". I think that wouldn't lead to universal hatred. People would research whatever caused the effect, and try to come up with a solution for it. And if not, they would make friends via Skype. And if the effect was planet-wide, their societies would learn to deal with the effect before they met the other race.$\endgroup$
– user7868Feb 2 '17 at 10:24

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$\begingroup$The only thing that can rationally justify this kind of hatred is if there's some unavoidable reason why members of one race would want to harm members of the other, for example they ate eachother. Although even then, that only justifies eating eachother, not eating eachother while scowling and thinking about how much they deserved it. I think the fundamental problem is that you want to rationally justify hatred, something which is intrinsically irrational.$\endgroup$
– Jack MFeb 2 '17 at 11:58

$\begingroup$To expand a little, you could have two races, where one is... basically just normal humans, and the other simply has an ashen skin color, maybe because they're normally in the cold and have slightly evolved to have more constricted blood vessels "by default", which would keep them a little warmer. When you die, all those muscles loosen, leaving you looking less pale and ashen, so the "normal" humans look like corpses; likewise, when a normal human dies, they become pale because nothing's keeping the blood moving, so the cold people look like dead bodies to normal humans.$\endgroup$
– Nic HartleyFeb 1 '17 at 19:40

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$\begingroup$@QPaysTaxes Hmm, this might lead to interracial necrophilia as a plot intrument. In fact I can see a variant of Romeo and Juliet playing out in my mind right now.$\endgroup$
– KalleMPFeb 2 '17 at 5:55

As presented, you show that you don't actually understand tribalism, racism, sexism, or pretty much any other -ism. Let me explain, but don't look for politically correct terms or comfortable lies.

Systemic racism comes from one of many sources:

Endemic social pressures. (If your child doesn't have certain traits, they will not be 'desirable' mates, and thus your genes will die out)

This is true of pretty much everywhere on the planet (African, India, Asia, North America) where there is a socio-psychological pressure to have 'lighter-skinned' offspring. The unwritten 'rule' is that people with lighter skin are 'more attractive', 'more trustworthy', and 'a better pick for a life partner'.

Lack of social interaction. (US versus THEM, tribalism)

Any tribe has learned, as a group, that what they do not know can kill them. Don't think modern times, think stone age (you know, when our instincts needed to be sharpest in order for our species to survive: you've never seen a lion before, that's a lion, KILL IT!) This brand of racism (tribalism) comes from the ingrained, almost pathological, need to protect what you know and love from what you do not know or do not love.

Lack of knowledge

Ignorance is the source of much of the world's problems. If I know nothing about you, and I meet you for the first time in a situation that causes me to be cautious (like two groups from these two civilizations you speak of), you and I will both be wary of each other (as explained in my previous point). I have no reason to be nice to you, you have no reason to be nice to me. If we are both brought up in the same way (meaning both are brought up to be friendly and accommodating, even with people we do not know) then there will be no problem. However, if you are brought up to be accommodating, and I am brought up to ignore or take what I need, then we cannot possibly understand each other. The lack of knowledge will cause friction, and that friction will not lead to anything good.

Competing for resources. (This rock isn't big enough for the both of us)

Another very real option is scarcity. If there is only enough for one of our civilizations to survive, then I'm going to kill you to make sure my wife and kids have a better chance. Some might disagree and argue that human nature would lead us to cooperate (and there are some psychological and sociological studies to back this up). However, if that were the case wars would cease in our world... That isn't the case, last I checked.

Pride. (I am important, therefore you cannot be)

This one is dependent on how the peoples are raised, both as individuals and as a tribe. If you have two groups believing they are each more important than the other, then they will not care how they treat the other. But this boils down to the individual, because if the individual is taught "A man should not be judged by how he treats his superiors, but rather how he treats those below him", then even if one feels superior, hate will not be bred between the groups for that reason.

That means the source of this rift, as you desire it, MUST stem from: limited resources. Because every other option goes against your stipulations, or doesn't require a deeply seeded 'hate'. Though I still feel it isn't needfully 'hate', more a love for someone that needs the same things you are trying to take from me. For a real-world example, see Feudal Japan.

$\begingroup$So like the Nazis and the pure 'master race'.$\endgroup$
– flithFeb 3 '17 at 12:06

$\begingroup$@flith Yes. It's things like that that are used to stir people, to give them a reason to justify their hate or disdain. Because of that, if you do not agree, you or your descendants will be ostracised because you/they aren't 'pure', like purebloods in the Harry Potter series.$\endgroup$
– Fayth85Feb 3 '17 at 18:43

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$\begingroup$Not sure about the “you don't understand anything“ but then a really good answer$\endgroup$
– Felix DombekFeb 4 '17 at 2:58

Odor would probably be the easiest way to justify an enduring race hatred. Just as there are pheromones, which induce attraction within those who smell them, there are probably also anti-pheromones which induce negative emotions and even hatred.

Make your blue skinned people's pheromone an anti-pheromone for the orange skinned people and vice versa. Then limit both races access to soap and perfumes.

$\begingroup$This seems like a cause of racism rather than a justification.$\endgroup$
– ShufflepantsJan 31 '17 at 20:50

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$\begingroup$@Shufflepants, The OP is a world-builder. From that perspective, what is the difference between a cause and a justification. I know that for the people of each race in the story, justification of why they are prejudice might be different from the actual cause. But I don't think that is necessarily true for a worldbuilder.$\endgroup$
– Henry TaylorJan 31 '17 at 22:53

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$\begingroup$That would also be a cause to start the process of speciation.$\endgroup$
– JDługoszFeb 1 '17 at 7:26

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$\begingroup$You could even go farther than odor, and have each group emit some chemical which is mildly harmful to the other group. Thus they could tolerate each others presence for short times, if needed, but in the long term it would be better for their health to maintain separation.$\endgroup$
– joeytwiddleFeb 1 '17 at 8:32

$\begingroup$Most likely cause. "Race" implies geographic isolation to inhibit interbreeding. A (geologically) recent mutation could affect a pheromone that simply forces them to avoid each other, even if they can't actually say why.$\endgroup$
– user2338816Feb 2 '17 at 8:20

Controlled release of a Hate gland

Ours is a rational species with a long history, and a wealth of medical knowledge. We've applied it mainly to ourselves. We have our love, our hate, jealousy, hope, like you. But unlike your mysterious human minds, each of our brains is just one big set of glands. When the Happiness gland fills, it needs to be released. When the Sad and Pity gland fills, we know it too, and we watch your Earth, the better to release those emotions.

We also have a Hate gland. We discovered it long ago, and its discovery has prevented us from destroying all life on our planet. Plus our solar system. Also your puny Earth. Because our Hate gland fills, too, and it tells us to Hate. All the time.

How do we release the burden of the Hate gland safely?

We hate each other. So, I hate Alien Greens, for the better of our world. There's one passing right now. Watch me stare at him, hatefully. They understand it of course, and they hate us Alien Oranges right back.

I hope you understand my justification. Farewell, and enjoy your stay. You can get back to the transports if you just keep right along that Orange Wall. Also, perhaps you should stay clear of that Alien Green, now that he's seen you talking to me. Such is life here on Planet Powderkeg.

Predator vs Predator historically

You have two sentient species... but maybe they used to be a hunter/prey relationship. Both of them used to eat the other for dinner. They are well past that today, both feeding on other prey species, but in the back of everyone's mind, everyone knows that the other would taste REALLY GOOD. And every few years, some whack job tests the theory out and, sure enough, he/she reports that the other species is The Best Thing Ever Tasted. The whack job gets tried for murder, and convicted, but that just adds to the constant tension.

This is a bit of a stretch, but could this "racism" be the result of years and years of deliberate propaganda by the respective governments of each side? In the same way that a prolonged marketing campaign has led us to associate diamonds with love, romance and value (spoiler, diamonds have nothing to do with the above), the governments of each population could have spent years educating their people to hate the other side, to the point that it has now become ingrained and instinctive.

A possible reason why each government would want to do so could be to establish a "common adversary" and thus enforce unity within their own community. It sounds like each population is self-sufficient. In this case, such a scenario could be the result of collaboration between both governments (for simplicity henceforth Red and Blue), along the lines of "I'll teach my Reds to hate on your Blues (and vice versa) so that my Reds will stay loyal to ME as long as I unite them against the Blues." This is a bit like the "War is Peace" argument from 1984.

$\begingroup$The thing about this is that a government propaganda campaign is very unlikely to affect every member of a human-like race. Most of them, sure, but there can be expected to be some strong skeptics.$\endgroup$
– David ZFeb 1 '17 at 17:13

$\begingroup$@davidz: I agree, but I also think that would hold true for more or less any reason, even the biological ones.$\endgroup$
– XenocaciaFeb 2 '17 at 1:28

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$\begingroup$+1 for the diamond propaganda shout-out. Sounds too much like our earth, however I agree with @DavidZ, it would not be universal and would not cover the young, simple minded and the critical thinkers.$\endgroup$
– KalleMPFeb 2 '17 at 6:05

Differing Values

The Dune series by Frank Herbert imagined some serious racism between the Tleilaxu and other groups, although all were humans. Some of this was based on physical appearance, but there were also a large part of it based on the different values of the groups.

For example, the fact that they never saw Tleilaxu women was concerning to other races (leaving aside the rumours), and if I remember correctly the Tleilaxu were equally offended by the other races permitting their women to wander freely in public.

We also see this in the real world. Sometimes we find people who dislike other groups not because of their physical characteristics, but because of their differing culture.

"The way they treat animals"

"The way they treat their older family members"

"Their obsession with money"

"The way their women dress", etc.

So all you need is groups with differing values, and you have a reason.

Justification

But to make it strongly justifiable, I think you would need to demonstrate some measure of harm.

Some of these problems might be reconcilable, if (individuals from) the two groups wanted to work on them. Others might not.

Existing Conflicts

There are a multitude of situations in the real world that you could look to for inspiration. Differing religions (the conflict in Ireland, or the one in Palestine-Israel), competition for resources, or indigenous people versus colonists.

Feeling uncomfortable is all you need

You seem to want some built in difference to cause this, several examples spring to mind: left and right handedness, height, body shape, face shape but I'm sure there are many more.

For handedness and height it could be that every time one visits the city of another they feel uncomfortable. Stooping through doors or having people tower around them, small things like day to day objects not being where they expect (handles on toilets for example). Body shape would mean the chairs in another city are uncomfortable. There are many examples you can think of. If one people are constantly made to feel uncomfortable when visiting another they will be on edge even if they can't really put their finger on it.

Face shape is another possibility, perhaps they each find expression of emotion hard to read on the face of the other...this is going to make you feel uncomfortable too.

Basically racism isn't founded in on some well thought through emotions but on a feeling of discomfort, something that makes you prefer you and yours over 'them'. This divide is the basis for all racism but above are some ideas of how it may be perpetuated.

nearly all human societies have a strong taboo against mating with family members. this can be easily explained by the dramatic increase in birth defects in the offspring of close relatives (e.g. siblings, parent/child). this is due to the negative effects of various recessive genes and the fact that the entire human race is extremely inbred (e.g. compared to chimpanzees). obviously, close relatives don't usually hate each other, but that's also explained by our genetic imperative to help our close relatives reproduce. so, the question becomes: what if there was a group of people that my people liked, but almost certainly would cause birth defects if we mated with them. if so, we would evolve lots of aversion mechanisms to ensure that never happened.

let's say at some point in the history of this species there was a mutation in a gene for forming skin. anyone with 2 copies of the "normal" version are fine. anyone with 2 copies of the "mutated" version are fine. but if you have 1 copy of each, your skin constantly peals off painfully while excreting a stinky green goo. how the first person with this mutation managed to reproduce at all is a mystery, but i'll assume he was not a nice man. in any case, his descendants eventually formed their own community. once that community became isolated from the "normal" community, they eventually all had 2 copies of the "mutated" gene, so they looked normal as long as they didn't interbreed with the "normal" population. fast-forward a few thousand years and both populations have developed strong social, political and genetic deterrents from interbreeding (e.g. taboos against talking to "others", laws forbidding interaction and hormonal repulsion making "others" stink). no one can remember what happens when they interbreed because it never happens. perhaps in the last few thousand years both societies have developed genes to handle the "mutation" without any ill effects, but the secondary repulsions remain. perhaps even the hormonal repulsion has eroded away, so they smell fine to each other. now, you have two societies that could interbreed (and otherwise interact), but simply choose not to for historical reasons.

$\begingroup$The lethal aspect of the half mutant could have eveolved a bit later by chance after the populations were already diverged (on two continents), never to be able to interbreed again. Basically if the birth defect had moved from strong propencity to certainty then the taboos would become total even if ships were rediscovered.$\endgroup$
– KalleMPFeb 2 '17 at 6:18

Modern, globalized culture as we see today is kinda new. Back then being from a different clan or tribe would be enough to get yourself shunned from society, even if you had the same skin color or general appearance of the people you're trying to mingle into. But if you were different, chances are you would end up having a really, really bad time. Just check all the fuss we still have because of stupid things like skin color.

Just give your races some sort of different skin or hair color, or maybe even some sort of different type of hair. Make then "Blues vs Oranges", or "Curly Hairs vs Balds".

Those types of differences shall be enough to explode a society if their leaders are fanatical enough.

If you don't want to make the difference on your societies being "flavor", use their country of origin as a basis. Civilization A always lived on the country, while Civilization B is a newcomer and grabbed a slice of land for themselves.

So, you have a conflict over land that, given enough time, would evolve to racism.

But really, you don't need much to justify your people being racists than just picturing them as human-like. We are that bad.

$\begingroup$You don't even need "racial" differences. In Ireland, people know to identify someone as Catholic/Protestant, street gangs can ID Blood/Crip etc, football fans can divide into Arsenal/Chelsea or Auburn/Alabama. We really are that tribal.$\endgroup$
– Monty HarderFeb 1 '17 at 18:57

Races are socially accepted groupings of genetic traits that distinguish those who have those genetic traits from those who do not, typically limited to genetic traits outwardly observable, such as facial bone structure or skin color.

However genetics carries more than looks, including aggression, certain mental illnesses, and aspects of intelligence. If traits such as these were more pronounced between the socially constructed groupings (resulting likely both from a biological side--how their genes interact--and a social one--both how the races are defined and how culture evolves within the races), the biologically-driven foundation of personalities and world views becomes more and more pronounced.

If one race abnormally highly suffered from schizophrenia, another race had abnormally high levels of autism and low intelligence, and another race was extremely aggressive at a biological level it'd be very easy to form racism grounded in scientific biological facts.

This strikes us as wrong as humans, since we don't see these traits concentrated in one race or another to any socially-defining degree. However if one race is biologically drastically more prone to have X or Y negative personality/mental trait, racism becomes...

...Well I don't want to say "justified," but it does seem more justifiable.

What if each race's biological functions require (similarly to humans) that certain kinds of bacteria exist in their systems? The catch of course is that the bacteria species A needs to survive actively attacks species B, and vice versa. Thus if members of the two species are brought into close proximity for lengthened periods of time, members of both species become ill and each species blames their sickness on the other.

Thus the children of each species are taught to hate and avoid the other species for their own safety, and the cycle continues.

In addition, the exact functionality behind this reaction could also only be ascertained from a full examination of the other species (e.g. by autopsy) and the person performing the examination would have to be protected to prevent being affected, which would make it particularly hard for the species to fully understand the situation, hence why the behaviour might persist for a great many years.

Your premise that it can't be natural is invalid. Simply saying someone is rational doesn't mean they can overcome their baser evolutionary drives. Take tribalism: it's the reason we look for people who are like ourselves, and it has a large part in the existence of nearly every prejudice, from political bias to racism. Simply being different in some way is grounds for some caution or even outright fear or hatred, from an evolutionary perspective.

As for the justification, it could be any odd lie about the other race. "They're violent, they'll rape our daughters, they're a corrupting influence that should not be approached under any cirumstances", would all be "justifications" for racism.

$\begingroup$"Take tribalism, it's the reason we look for people who are like ourselves" According to who? One of the strongest "baser evolutionary drives" is the desire to strengthen the gene pool by introducing variety. It's why so many people (of every race) see potential mates of other races as "exotic" and extra-attractive.$\endgroup$
– Mason WheelerFeb 1 '17 at 21:37

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$\begingroup$"...so many people" is still a minority. Most people tend to stick to their own group. We're born to parents of our own race, and whatever cultural values were passed down from their ancestors generally gets passed on to us as well, and culture is an excellent instigator for intergroup conflict.$\endgroup$
– ArborianSerpentFeb 2 '17 at 15:58

There are many variants (evidenced by the TVTropes examples) but the basic premise is that the knowledge that one is a clone can lead to a wide range of psychological issues dealing with the ensuing identity crisis.

Being a Clone Sucks -- Uncertainty Edition

One of my favorite takes on this is when the cloning process somehow leaves doubt as to which individual is the original and which is the clone. Each wants to believe that they are the original (without evidence suggesting it), mostly to side-step all the nasty implications of being a clone.

However confident an individual may appear, the nagging doubt that they may not be real is always in the back of their mind, and unlike their "definitely a clone" counterparts, it's unlikely they'll be able to accept and get over it.

Why Uncertain Clones Hate Each Other

This also naturally leads to negative emotions between potential clone counterparts. Let's assume the following axioms and see what follows:

I want to prove that I am real

Only one of us is real

(2) directly implies (3)

Any evidence supporting the hypothesis that I am real also supports the hypothesis that he is fake

It's easy to rewrite (1) and (3) to:

I want to prove that he is fake

(4) From rewritten my counterpart's perspective:

He wants to prove that I am fake

(1) and (5) are directly at odds. We have a source of conflict. We can both be rational, and we can both be ethical, but as long as our desires are mutually exclusive we will find ourselves on the opposite sides of things quite often, and that can bleed through.

If we add the following assumption:

clones are commonly expected to be inferior in some way to the original (more susceptible to disease, not quite as strong, not quite as smart, etc.)

there are a few more consequences:

any time I outperform my counterpart, I'm relieved and happy: This is evidence that I am real and my counterpart is fake. This could even lead to pity, which may be resented by my counterpart.

And it's corollary

Any time my counterpart outperforms me, I'm forced to confront the possibility that I might be fake again. This is unpleasant and uncomfortable. This always occurs just as my counterpart is experiencing the comfort and joy that comes from being secure in his reality.

A pattern emerges where my happiness is correlated negatively to my counterpart's happiness. The strength of the correlation could be strong or weak depending on just how much our sense of identity is important to us vs. our other sources of happiness, but it will always be negative.

Making it Societal

Possible Origin Story:

RedRace created Clones for slave labor. They ensured that they could not reproduce with an Original to prevent the nasty question of what rights should go to individuals born to one RedRace and one Clone parent, but otherwise left the genome as unaltered as possible, so they could easily be a drop-in replacement for Originals in every sector of the economy (even traits like inbuilt servitude might prevent them from functioning in some roles or functions). Clones, as a result of the slight genomic alteration, also ended up with Blue Skin, and were thus dubbed BlueRace. BlueRace didn't like the current world order, revolted, and during the war captured and deleted all records which proved RedRace was the OriginalRace, and started an intensive propaganda campaign claiming that BlueRace was the OriginalRace. Hundreds of years after the end of the war, no one alive knows the truth for certain. Almost everyone agrees that regardless of the truth, both OriginalRace and CloneRace individuals have the same set of rights and responsibilities, and should be treated as equals. Despite nominally open borders, it's far more comfortable to avoid being reminded that someone is fake (especially without members of your own race near you to back you up) by going into OtherRace territory and interacting with members of OtherRace.

Societal achievements could take the place of personal achievements for purposes of determining which is real, spurring advances in technology, governmental systems, or ethics.

Individual achievements may still be seen as supporting evidence for one race's superiority, especially among the less "enlightened" members of the species. Sports and other competitions would be extremely tense for these reasons, especially any sport which involves referees for scoring or rule enforcement like gymnastics or even football.

Any achievements made by RedRace will be attributed to luck, foul play, or just "not being that important" by BlueRace, while they will emphasize any area where they are outperforming RedRace.

Bonus Round

If religion teaches that only "God's creations" have a soul and get to enjoy the afterlife, being real has much stronger implications and the tension goes up a lot, even if both races follow the same religion, and even if the religion preaches tolerance and love for every living creature, clones included!

RedRace zealots could word tirelessly and donate all wordly goods towards helping BlueRace people live comfortable and fulfilling lives, on the premise that they have the entire afterlife to enjoy themselves, BlueRace only has their 80(?) year life. They legitimately practice love, compassion, and self-sacrifice, but it's rooted in the assumption that RedRace is real and BlueRace is fake (and somehow inferior) so it ends up being divisive all the same.

In real life, among human beings, very little actual rational justification is needed for the development of lasting and consequential racial animosity. In fact, the animosity is sometimes sharpened when the groups are actually very similar from objective standpoints. Notable current and historical examples include the Jews vs. the Samaritans, India vs. Pakistan, Hutus vs. Tutsis, the Serbs vs. the Croats, Korea vs. Japan and the English vs. the French.

In a famous experiment, a schoolteacher successfully and reliably elicited typical racist behaviors from a group of schoolchildren simply by dividing them into two groups based on eye color, and favoring one group over the other.

For your book, I would suggest some historical circumstance in which one group was favored for some trivial trait. To make the hatred especially sharp, let the other group have now gained the upper hand. Black Skins, White Masks might be a good resource for you to read.

I think it would be just enough for a more or less literal racism if biological differences between the two made it that one of the races was strictly carnivorous and the other strictly herbivorous. The difference could be the effect of a virus changing their digestive systems. Originally, both races were omnivorous, much like humans, but they were different enough so that their organisms responded differently to the same virus. So now it's easy to distinguish between the two even though the difference in their respective visual traits has nothing to do with the main problem.

Since their diets are so radically opposite, it's very difficult for both races to live in the same place. And if they have little opportunity to socialize with representatives of another ethnic group, it's very easy to perceive them as a threat. False stories start to spring up. The other culture starts to look extreme and barbaric. And so on.

The arguments against carnivores are pretty simple: They feed on carcasses. They require slaughter houses. They (everyone knows that) drink blood. They are treacherous and cunning: after all they raise cattle, acting as if they were friends to the innocent animals, and then they kill them. And what if... what if they eat us, herbivores, too?! There are stories about missing children, you know.

It's a bit harder to think something out against herbivores, but this is not impossible. Carnivores are shunned and hated by the herbivore society, so they tend to come up with ideas which show that in fact the carnivores are better. Herbivores are weak. They prefer shady politics instead of a simple, honest conflict. They have no honour. Long time ago everyone we were all carnivores and only they changed and now they lie that we were changed too. When a carnivore mates with a herbivore, the child dies. See? (You can elaborate on that: what if living or dying, or the diet, of the offspring depends on who is the father and who is the mother, or on the sex of the child?) On top of that, herbivores are hypocrites. How do think, why there is no cattle in the herbivore lands? What happened to all the animals? Huh? Huh?

The reason that racism is problematic in our world is that it's based on a lie: that people of other races are inherently different, in a way that makes it harmful for them to interact with us. The simple truth is, we're all human and we all have approximately the same capacities for good and for evil as everyone else.

If you want it to be justified, take that lie and make it true in your world. There is something inherent in their nature that makes at least one group, if not both, fundamentally harmful to the other.

Subtle biological changes can exist between human subraces. Caucasians are almost unique in their ability to tolerate lactose beyond childhood. Native Americans don't have the genes for facial hair. IIRC the Japanese have an enzyme that improves their ability to digest raw fish. Sickle-cell anemia is common in people whose ancestry traces back to parts of Africa in which malaria is endemic, because the altered blood cells are resistant to it. And so on.

Perhaps in your world, race A has a pheromone that is harmless to them but triggers allergic reactions in many members of race B. Now it is literally not safe for them to be around each other. That's all it takes.

I think the key to inflaming racism is, counter-intuitively, denying that racism exists.

People often conflate racism with celebration of diversity. Little Italy, Chinatown - these aren't racist vestiges, these are celebrations of communities that have naturally aligned themselves along dichotomies like race, language, culture, etc.

People naturally organize themselves into teams, into sides, etc. It's a political calculation. Together, as a community, you can vie for jobs, neighborhood development, other things. The key isn't to ignore race (which would simply strip less powerful communities of their ability to organize politically), but rather to integrate multiple communities into a cohesive functioning society where race is a basis for cultural commerce instead of conflict.

However, in your society, for whatever reason, multiculturalism is anathema. There isn't even an acknowledgement of culture, or a concept of race. They're simply intent on steamrolling each other until the one "true" culture dominates.

In essence, it has to be a psychological change. Trying to actualize race via outward biological devices undercuts its core abstractness. Race may seem irrational to you, but I'm reminded of what Douglas Adams wrote about horoscopes:

In astrology the rules happen to be about stars and planets, but they could be about ducks and drakes for all the difference it would make. It's just a way of thinking about a problem which lets the shape of that problem begin to emerge. The more rules, the tinier the rules, the more arbitrary they are, the better. It's like throwing a handful of fine graphite dust on a piece of paper to see where the hidden indentations are. It lets you see the words that were written on the piece of paper above it that's now been taken away and hidden. The graphite's not important. It's just the means of revealing the indentations. So you see, astrology's nothing to do with astronomy. It's just to do with people thinking about people.

Keep it simple. A natural phenomena has caused members of the same species to evolve some physical trait that is mutually disturbing to other group. Pick one or more of:

1.) Each group smells positively disgusting to the other group due to some environmental change in their body odour. Being around them is literally nauseating.

2.) Same concept but the other group looks positively ugly or otherwise offensive (too bright, too dark, too purple, too trapezoidal, whatever). It literally hurts to look at them.

3.) They sound sooo irritating. Listening to them is like nails on a chalk board. They are just too damn loud, or too quiet, or their accent sounds like childish gibberish.

The traits need not be so extreme as to make them a different species for the two groups to be mutually upset by the presence of the other. For 2 and 3 even distance is no cure - audiovisual communication would remain an unpleasant chore that would hardly create a friendly relationship.

If you want something more technological it could be as simple as each race has a stash of WMDs. They constantly live in fear of each other.

If you want something more unusual consider an evolutionary change that caused the blood of each group to change chemically into something dangerous to the other. Maybe one group has acidic blood and the other group is highly basic, so close contact causes burns. Or maybe in the territory of the other group people have a reaction with the air that causes them to randomly spontaneously explode if they aren't wearing special containment suits.

$\begingroup$This reminded me of the original Star Trek episode: Let That Be Your Last Battlefield where the last two of their "races" had their skin divided equally into black and white, except one was a mirror image of the other.$\endgroup$
– delliottgFeb 3 '17 at 0:00

Incompatible biologies including each species associated ecosystems such that the systems nor the sentients can share the same area and when one ecology expands the other must shrink, making it a zero sum game.

Chirality would work well to create such immersible yet otherwise identical ecosystems. One species would be levirotary the other dexter (left or right.) They couldn't coexist in the same ecosystem because some of the enantiomers would be toxic to the other side. Otherwise, they would look and act identical. The sentients could not eat the same foods from the same cropland nor intermarry, It would be a true US vs Them, winner take all, planet wide, ecosystem anchored, war to the knife.

The only hitch is that it wouldn't really be racism. We're all taught that racism is irrational but for much of the past 500 years it's been quite rational, judge by the behaviors it evoked even though the fictional rationale kept shifting. Racism is a fictional construct with a functional purpose. It creates a sense of genetic-kinship within a group who are not actually related beyond superficial appearance and/or living in proximity. That sense of kin-indentity was need to form larger and more cooperative units.

In the middle-ages anywhere in the world, if you told a noble they were in anyway genetically related to the peasants who tilled their fields, the noble likely would have literally kill you for the insult. A couple of centuries later, they would have replied, "well we are all English, French etc and then eventually "white."

Yep, the distant past sucked so bad that what we today regard as one of our greatest evils, represented at the time, a big step forward in the moral advancement of humanity.

Counter-intuitively, the more egalitarian and democratic countries became, the more they needed that sense of collective identity, and the stronger racism grew. Eventually, though the merit driven culture triggered by the evolution of corporations, destroyed the functional basis for racism just as it had for class (it's not just coincidence that the anti-slavery movement was almost entirely comprised of Corporate elites e.g. Hosiah Wedgewood.)

The problem with many parts of the world today, is that they haven't progressed up to racism yet and remain stuck in seeing only their extended families as being moral equals to themselves.

The problem with the Chirality scenario is that each side would present a real material danger to the other side so their "racism" would not require a fictional justification. The justification for hating the other side would be quite real and rational, in terms of it being rational to preserve one's own life, and lives of one's family over the lives of others. Ruthless, but rational. Call it Darwin's diabolical calculus.

Not sure its what you're looking for but it's the best I can think of that matches your criteria.

When you get right down to it, you're trying to (if I understand the question correctly) come up with a rational justification for hatred, which is an emotion. You can't rationally justify an emotion, emotions are rationally neutral. The reason is that the way you would normally justify that some course of action is rational or irrational is to examine its effects on the world - if the action will lead to a positive outcome (for some definition of "positive"), then it's rational. Emotions in and of themselves have no effect on the world. You can say that some action motivated by an emotion is irrational, but then you're not discussing the emotion anymore, just that action. It would have been irrational no matter what motivated it. The best you can say is that since hatred feels bad, it's irrational to feel hatred, but then that's a bit like saying it's irrational to feel pain.

What you can try to justify rationally are some of the symptoms of racial hatred, ie some of the actions that hatred often motivates. Segregation, abuse, war, genocide. Why would it be rational to do any of those things?

At the basic level, in order to justify that anything is rational or irrational, you need to have a value system. Like I said, to say that something is rational is to say it will lead to a positive outcome, but the definition of "positive" is arbitrary. For humans, for example, we tend to assume axioms like "dying is bad" and "physical pain is bad". If your two alien species had biologically in-built value systems that conflicted with one another, then it might seem rational for one to exterminate or at least avoid the other.

But wait, you say. Surely if these are perfectly rational beings, they can see that their value systems are fundamentally arbitrary? Surely they can see that they shouldn't despise each other because of what kinds of likes and disgusts their biology happens to have built into them? Well, yes, if you boil it right down, maybe these beings aren't perfectly absolutely rational. But consider this: can you rationally justify that murder is bad? Is it? If no one knows or cares about the victim, at least? If they don't suffer while dying? Who have you hurt? They're not around anymore to care that they're dead. The fact that we value not killing each other is just a (evolutionarily sound) biologically innate value that human beings have. This is the kind of thing I'm talking about. Not values like "women shouldn't go to college", values like "eating cake is better than being tortured to death". This is the same as @joeytwiddles answer, but on a much deeper level.

You need two species to have values that satisfy four conditions:

They are absolutely biologically innate on the same level as the human aversion for death and suffering.

It's impossible to build a society in which both sets of values are satisfied at the same time.

It's possible to build a society in which either one of the sets of values is satisfied, so that you aliens could at least have their own societies and civilizations.

It's plausible that a species could have survived natural selection with those values.

As an example, I don't think it's difficult to imagine a species who don't have a problem with torturing and killing people. Imagine a kind of an ant-like species in which each individual only values advancing the population as a whole, and doesn't actually have any fear of death or pain, or a tendency to emotionally attach themselves to individuals (a.k.a. they don't have a concept of friendship or love). Evolutionarily, this is just taking quantity over quality. This species would have no problem with mass murder, probably including of other species. How well do you think we'd get along with them?

They are more enlightened now, and just came off a long period of peaceful cooperation.

The races must be kept separate, because intermarriage delivers a lethal combination of genes to offspring.

Each independently discovered a technologicAL means of seeing the future, albeit fuzzily.

In the future they see, one of the races has been apparently exterminated.

The truth is, a mutagenic virus caused changes to the "exterminated" race that reversed the mutations that made them distinct, causing the races to merge, so really no one was exterminated, but tell them that after the war that kills most of both populations and creates the virus in the first place. Self-fulfilling prophecy.

$\begingroup$+1 for point 3, it is all that is needed. Any conception across the race barrier causes the death of the mother or the birth of a 'monster' and there is no cure. Over time the avoidance would be systematic and entrenched into every social, religious, legal and educational avenue.$\endgroup$
– KalleMPFeb 2 '17 at 6:11

Race 1 will view Race 2 with suspicion, unease, and just plain fear / hate. They think they are better than us. They are so creepy. They are an abomination. Why do they need a plasma cannon grafted to their torso unless they intend us harm?

Race 2 will view Race 1 as a relic. They are barbarians. They refuse to transcend with us. They think they are so much better. They are just so disgusting with those fleshing bits. The constitution gives me the right to free expression and if that means adding tentacles to my arms then I will add tentacles to my arms!

Maybe it is something as stupid as one society believes that the best way to further species progress, it would be to do stem cell research. The other society, who is more grounded by natural means of progression may find stem cell research to be too dangerous and could harm the genes. This breaks into fights and difference of opinions that lead then to years of hostility.

The very definition of racism is the belief that one race is superior to the other. Maybe it is the belief that This race of XXXX, which has horns, is much more appealing and gives them the feel of the alpha race over the others. Could be something as silly as one race has hair and the other doesn't which in their eyes makes each other look ugly.

One thing you shouldn't do and I would probably re-word the question is that you aren't justifying racism, but rather providing a reason it exists. Justifying it would mean that this behavior is okay. Depending on your world's set of values, racism isn't something that you justify and validate and say "yep, you are right, their lack of horns is pretty beta. You have every right to be jerks to them". Within the races, they may justify racism to themselves for their reasoning, but to the readers and the rest of the internal world species seem very silly. Depending on where you are trying to go with the story, you will more than likely want to, at some point, show some form of realization that the justification they held was wrong as an attempt of character growth.

E.g. sexism: one of the races is intensely misogynistic, and treats their females with disdain, scorn, dismissiveness about sexual assault, and only pays them 74% of what an equivalent male would be paid; whereas the other race has deep-seated misandry, and like the mythical Amazonians subjugate their males nearly as slaves.

While each race "accepts" that the other race functions as they do, and realize that they cannot change things - they are still offended by their racial culture, even repulsed by it.

Here's another idea, because I think that one is too realistic for you.

Each race has a certain magnetic alignment, giving off a slight, but significant, charge of magnetism.
The key is that each race is aligned opposite to the others.

And while "opposites attract" may often be true, in this case the opposing magnetic charge actually makes the other race physically queasy, like deep down in their gut. They are literally sick to their stomachs from being around the other race.

They probably don't even understand this cause, and maybe don't even realize they are affecting the other race too. All they know is they are affected negatively when the "others" are around.

If you could have them at significantly different altitudes, that could work.
One race lives in the deep valleys and low plains of one continent. The other lives in the high mountain valleys and plateaus of the second continent.

The lowlanders would have trouble moving about in the highlands, due to the thin atmosphere.

The highlanders would be very uncomfortable in the lowlands because of the high humidity.

You'd probably need an altitude difference of about 12-15000 feet to get that kind of effect, but it's nothing a creative geography can't produce.

Someone above mentioned pheromones. What if one of the races developed a pheromone which they can release as a conscious action, which makes the other race do their bidding? I'm thinking the pheromone acts like an airborne version of sodium pentethal. Obviously, the natural thing for the Affected race to do is to develop gas masks, from a very early point in their tool use, but for both races on this planet, pheromones are an important method of communication, and they don't want to wear these gas masks all the time. Eventually, the Excretor race promises not to use that particular pheromone when an Affected is around. But this pheromone is important to the Excretors' communication, and they resent not being able to use it. They are allowed to use it in sealed-off spaces (in a city full of people with a scent-based communication system, these are very important) that are designated as Excretor spaces, and as a gesture to this, public buildings and transport often has an Excretor space and an Affected space.

I think this would work better if the Affected are in power, but hate the casual way an Excretor can get control of them, while the Excretors resent the way the Affecteds impose on them and deny them their ancestral rights because the Affecteds are weak and couldn't cope if they did.

Or you could use the reason that we have in the real world: Political leaders trumpet the flaws, crimes, and dangers of the other race (inventing such flaws, crimes, and dangers if necessary) in order to manipulate their own herd, for the sake of influence and money.

Much of the racism we have today would simply die out if there wasn't someone, somewhere who was making hay out of it.

I could cite specific examples, but I don't want to start a fruitless debate.

Assuming that you are looking for "rational" reasons, you need a real issue that make both races incompatible. Something that not only makes them to keep distance but also to dislike the others. This is my proposal: mutual alergic reaction.

Each race is alergic to the skin cells from the other one, so they cannot be in contact. And, as most of the dust we find it our homes is produced by our skin cells (we are constantly renewing our skin), when one race has been in a zone for a long time, that zone becomes uninhabitable for the other, so they can not share resources like food, clothes, tools or any other hand-made thing. They are in a ethernal quarantine for each other, so they see the others as an illness (like zombies) that tries to expand their territory by "contaminating" the soil that they step on.

Thank you for your interest in this question.
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